The overall aim of this study is to determine, in a longitudinal fashion, whether there are race and sex differences in BMC and Ca metabolism from 3 to 18 months of age, and whether differences are related to physical activity. We theorize that higher activity levels result in increased osteoblastic activity, as indicated by increased serum osteocacin concentrations, and diversion of calcium and phosphorus from blood, resulting in an increase in serum parathyroid hormone and 1,25(OH)2D.